Construction of the vertebrate head requires the movement of several distinct cell populations in the anterior of the embryo. To investigate the genetic and embryological basis of vertebrate anterior morphogenesis, we will analyze the zebrafish maternal-effect mutant pollywog (pwg). pwg mutant embryos are patterned normally but display a robust, aberrant morphogenetic movement in which cells of the anterior mesendoderm plunge ventrally or are deflected laterally instead of moving to the appropriate anterior position. This defective morphogenetic movement has not been described previously, indicating that pwg will provide a molecular inroad into a previously unstudied facet of vertebrate anterior morphogenesis. The experiments described in these aims will allow us to perform detailed structure function analysis of the pwg gene and define precisely the cellular function of pwg in anterior morphogenesis. Aim 1: We will test the hypothesis that pwg controls the movement of specific cell types that form head tissues. We will assay the movements of anterior cells to determine precisely which cells are participating in the aberrant anterior movement. Aim 2: We will test the hypothesis that pwg functions to control specific cell behaviors. We will assay the rearrangement of tissues that participate in embryonic morphogenesis to detemrine if pollywog is required for a particular morphogenetic behavior. We will assay the behavior of specific cells to determine what specific cell behaviors require pollywog. Aim 3: Identify the pwg gene. Through a combination of positional cloning and candidate gene approaches we will identify the gene encodingpwg. The zebrafish pollywog gene is required to shape the embryonic head. The morphogenetic movements that shape the head in zebrafish and humans are similar and defects in shaping the embryonic head lead to craniofacial defects. Identification and characterization of the pollywog gene will allow us to assess the role of the human POLLYWOG gene in craniofacial and neural tube development. Ultimately this work will allow identification of genetic and environmental risk factors that lead to human birth defects.